This invention relates to a mode setting control system for a one-chip microprocessor to which a mode setting circuit is connected.
In recent years, semiconductor technology has advanced very much. Through these technical advances a so-called one-chip microprocessor was developed which comprises a microprocessing unit (MPU), a random access memory (RAM), a read only memory (ROM) and the like--all formed on a single chip. The one-chip microprocessor is set to one of several various operation modes so that the user may inspect the RAM, the ROM and the like, expand the external memory or an input-output (I/O) device, or supply either bus data or I/O port data to the connection terminals. To set the one-chip microprocessor to one operation mode, the following mode setting systems are selectively used:
(1) A system in which mode data generated by operating mode designating switches, provided outside the one-chip microprocessor, is selectively fed through mode input pins assigned to the various operation modes, respectively.
(2) A system in which a specific voltage (e.g. 12 V), other than the TTL level voltage (i.e. 5 V), is applied to one or more of the pins through which data may enter to be processed, thus supplying a selected mode data.
(3) A system in which selected mode data from mode designating switches is fed via pins, through which data may enter to be processed, when a reset signal reaches a threshold voltage utterly different from the TTL threshold voltage. And the threshold voltage of the mode input pins (e.g. V.sub.IL 2.2 V, V.sub.IH 4.0 V) is also different from that of TTL.
The above-mentioned systems (1)-(3) have inherent deficiencies however. System (1) requires many pins, and its cost is inevitably high. Further, since the pins are not used very efficiently, the one-chip microprocessor cannot fully function. System (2) needs a special power source (e.g. a 12 V power source) for setting the one-chip microprocessor to any operation mode desired, and furthermore data to be processed cannot be fed to the one-chip microprocessor as long as the specific voltage (e.g. 12 V) is applied to one or more of the pins to supply mode data. System (3) is disadvantageous in that the mode data entered is unreliable. The unreliability occurs because a mode signal is written when a reset signal reaches a specific threshold level (e.g. 1.4 V) and therefore, the mode data may be fed at the incorrect time if the reset signal contains much noise.
In short, each of the conventional mode setting systems contains at least one inherent deficiency.